A newcomer to the cantina in the pirate city of Mos Eisley in a galaxy far away would be confronted by a crowd of intimidating thugs: Rodians, Devaronians, Ithorians, Morseerians, Lutrillians, Siniteens and other alien buccaneers swilling liquor and picking fights while a band of hairless Biths play some kind of cosmic jazz.

Just another night at the Star Wars bar.

As exotic as they appear, these beings share a comforting familiarity: They are bilaterally symmetrical with, for the most part, a pair of eyes staring from a head connected by a neck to a torso equipped with matching pairs of limbs. They move across the floor on legs, not wheels. They make sounds with mouths and register them with ears. In their basic body plan they are little different from the motley crew in the public room of the Spouter-Inn where Ishmael meets Queequeg, Melville’s illustrated man.

Scientists have long debated how closely extraterrestrial life would evolve to resemble that on earth. Stephen Jay Gould, in his book “Wonderful Life,” took a contrarian view, arguing that with a slightly different roll of the Darwinian dice, earth would have been inhabited by creatures unimaginable.

Charles S. Cockell, an astrobiologist at the University of Edinburgh, is the anti-Gould. In his new book, “The Equations of Life: How Physics Shapes Evolution,” he argues for a cosmos populated, if at all, by anthropocentric creatures like those George Lucas dreamed up for the “Star Wars” films.

No matter how different the conditions on distant worlds, all presumably have the same laws of physics — from quantum mechanics to thermodynamics and the laws of gravity. And life, as Cockell puts it, is simply living matter, “material capable of reproducing and evolving.”

If there is biology elsewhere in the universe (and it has risen beyond the level of green slime) we would find it strikingly familiar, he proposes, not only in appearance but down to the carbon-based machinery in its cells.

Cockell’s book lucidly addresses biology’s great mystery: If we grant that life is an interplay of chance and necessity, in the words of the French biochemist Jacques Monod, then which has the upper hand?

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Monod came down on the side of chance, proposing that the cellular chemistry that seems so fundamental is just “randomness caught on the wing,” a grab bag of molecules that happened to be around when the first cellular membranes spontaneously formed, trapping a drop of the primordial waters. Cells with the fittest concoctions crowded out the weaker, but then the recipe was locked in — a frozen accident carried along for the evolutionary ride.

But Cockell argues that even at this deep level, the possibilities of life were tightly circumscribed. Rerun the tape of evolution, and DNA, RNA, ATP, the Krebs cycle — the rigmarole of Biology 101 — would probably arise again, here or in distant worlds. Single cells would then join together, seeking the advantages of metazoan life, until before you know it something like the earthly menagerie would come to be.

Darwin wrote of “endless forms most beautiful” (beauty being in the eye of the beholder) but Cockell makes a strong case that evolution is narrowly channeled. The endlessness is only in the filigree, like the spots on a moth or Queequeg’s tattoos.

In exploring the space of possible life-forms, evolution is no freer, Cockell implies, than the builder of a bridge. Consider the mole, a burrowing creature that is “an engineering solution to the compromises needed to effectively shift soil by maximizing the force applied over a small distance,” Cockell writes. “It is an organic manifestation of P=F/A.”

Pressure equals force divided by area. No wonder moles evolved independently in Europe (where they are kin to shrews and weasels) and in Australia (where their cousins are kangaroos and koalas). The contingencies of random variation are overshadowed by the hard reality of making a living in an unforgiving world.

That in itself is not a startling idea. The pleasure of Cockell’s book comes from the detail with which he pursues it. In a chapter called “The Physics of the Ladybug,” he carefully lays out some of the principles that dictate why such an insect almost had to be.

It would not be so surprising to find one crawling along the leaf of a potted plant in the Star Wars bar, oblivious to the surrounding noise.

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